1. Field of the Invention
This invention relates to non-invasive means of determining cardiac output or pulmonary capillary blood flow in patients and, more specifically, to partial re-breathing systems and methods for determining cardiac output or pulmonary capillary blood flow in patients.
2. Statement of the Art
It is important in many medical procedures to determine or monitor the cardiac output or the pulmonary capillary blood flow of a patient. Cardiac output is the volume of blood pumped by the heart over a given period of time. Pulmonary capillary blood flow is the volume of blood that participates in gas exchange in the lungs. Techniques are known and used in the art which employ the use of catheters inserted into blood vessels at certain points (e.g., into the femoral artery, the jugular vein, etc.) to monitor blood temperature and pressure and to thereby determine the cardiac output or pulmonary capillary blood flow of the patient. Although such techniques can produce a reasonably accurate result, the invasive nature of these procedures has a high potential for causing morbidity or mortality.
Adolph Fick's formula for calculating cardiac output, which was first proposed in 1870, has served as the standard by which other means of determining cardiac output and pulmonary capillary blood flow have since been evaluated. Fick's well-known equation, which is also referred to as the Fick Equation, written for carbon dioxide (CO.sub.2), is: ##EQU1##
where Q is cardiac output, VCO.sub.2 is the amount of CO.sub.2 excreted by the lungs, or "CO.sub.2 elimination," and Ca.sub.CO.sub..sub.2 and Cv.sub.CO.sub..sub.2 are the CO.sub.2 contents of arterial blood and venous blood, respectively. Notably, the Fick Equation presumes an invasive method (i.e., catheterization) of calculating cardiac output or pulmonary capillary blood flow because the arterial blood and mixed venous blood must be sampled in order to directly determine the CO.sub.2 contents of arterial blood and venous blood.
It has been shown, however, that by using the principles embodied in the Fick Equation, non-invasive means may be employed to determine cardiac output or pulmonary capillary blood flow. That is, expired CO.sub.2 levels, measured in terms of fraction of expired gases that comprise CO.sub.2 (f.sub.CO.sub..sub.2 ) or in terms of partial pressure of CO.sub.2 (P.sub.CO.sub..sub.2 ), can be monitored and employed to estimate the content of CO.sub.2 in the arterial blood. Thus, a varied form of the Fick Equation may be employed to estimate cardiac output or pulmonary capillary blood flow based on observed changes in f.sub.CO.sub..sub.2 or P.sub.CO.sub..sub.2 .
An exemplary use of the Fick Equation to non-invasively determine cardiac output or pulmonary capillary blood flow includes comparing a "standard" ventilation event to a change in expired CO.sub.2 values and a change in excreted volume of CO.sub.2, which is referred to as carbon dioxide elimination or CO.sub.2 elimination (VCO.sub.2), which may be caused by a sudden change in ventilation. Conventionally, a sudden change in effective ventilation has been caused by having a patient inhale or breathe a volume of previously exhaled air. This technique is typically referred to as "re-breathing."
Some re-breathing techniques have used the partial pressure of end-tidal CO.sub.2 (Pet.sub.CO.sub..sub.2 or et.sub.CO.sub..sub.2 ) to approximate the content of CO.sub.2 in the arterial blood of a patient while the patient's lungs act as a tonometer to facilitate the measurement of the CO.sub.2 content of the venous blood of the patient.
By further modification of the Fick Equation, it may be assumed that the CO.sub.2 content of the patient's venous blood does not change within the time period of the perturbation. Thus, the need to directly calculate the CO.sub.2 content of venous blood was eliminated by employing the so-called "partial re-breathing" method. (See, Capek et al., "Noninvasive Measurement of Cardiac Output Using Partial CO.sub.2 Rebreathing", IEEE Transactions on Biomedical Engineering, Vol. 35, No. 9, September 1988, pp. 653-661 (hereinafter "Capek").)
The carbon dioxide elimination of the patient may be non-invasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration, and is typically calculated as the integral of the carbon dioxide signal times the rate of flow over an entire breath. The volume of carbon dioxide inhaled and exhaled may each be corrected for any deadspace or for any intrapulmonary shunt.
The partial pressure of end tidal carbon dioxide is also measured in re-breathing processes. The partial pressure of end-tidal carbon dioxide, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PA.sub.CO.sub..sub.2 ) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (Pa.sub.CO.sub..sub.2 ). Conventionally employed Fick methods of determining cardiac output or pulmonary capillary blood flow typically include a direct, invasive determination of Cv.sub.CO.sub..sub.2 by analyzing a sample of the patient's mixed venous blood. The re-breathing process is typically employed to either estimate the carbon dioxide content of mixed venous blood (in total re-breathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial re-breathing) or determine the partial pressure of carbon dioxide in the patient's venous blood (Pv.sub.CO.sub..sub.2 ).
Re-breathing processes typically include the inhalation of a gas mixture that includes carbon dioxide. During re-breathing, the carbon dioxide elimination of a patient typically decreases. In total re-breathing, carbon dioxide elimination decreases to near zero. In partial re-breathing, carbon dioxide elimination does not cease. Thus, in partial re-breathing, the decrease in carbon dioxide elimination is not as large as that of total re-breathing.
Re-breathing can be conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. FIG. 1 schematically illustrates a conventional ventilation system that is typically used with patients who require assisted breathing during an illness, during a surgical procedure, or during recovery from a surgical procedure. The conventional ventilator system 10 includes a tubular portion 12 that may be inserted into the trachea of a patient by known intubation procedures. The end 14 (i.e., the end most distant from the patient) of the tubular portion 12 may be fitted with a Y-piece 16 that interconnects an inspiratory hose 18 and an expiratory hose 20. Both the inspiratory hose 18 and expiratory hose 20 may be connected to a ventilator machine (not shown), which delivers air into the breathing circuit through the inspiratory hose 18. A one-way valve 22 is positioned on the inspiratory hose 18 to prevent exhaled gas from entering the inspiratory hose 18 beyond the valve 22. A similar one-way valve 24 on the expiratory hose 20 limits movement of inspiratory gas into the expiratory hose 20. Exhaled air flows passively into the expiratory hose 20.
With reference to FIG. 2, an exemplary known re-breathing ventilation circuit 30 is shown. Re-breathing circuit 30 includes a tubular portion 32 insertable into the trachea of a patient by known intubation procedures. Gases may be provided to the patient from a ventilator machine (not shown) via an inspiratory hose 34 interconnected with tubular portion 32 by a Y-piece 36. Tubular portion 32 and an expiratory hose 38 are also interconnected by Y-piece 36. An additional length of hose 40 is provided in flow communication with the tubular portion 32, between the tubular portion 32 and the Y-piece 36, and acts as a deadspace for receiving exhaled gas. A three-way valve 42, generally positioned between the Y-piece 36 and the opening to the additional length of hose 40, is constructed for intermittent actuation to selectively direct the flow of gas into or from the additional length of hose 40. That is, at one setting, the valve 42 allows inspiratory gas to enter the tubular portion 32 while preventing movement of the gas into the additional length of hose 40. At a second setting, the valve 42 allows exhaled gas to enter into the expiratory hose 38 while preventing movement of gas into the additional length of hose 40. At a third setting, the three-way valve 42 directs exhaled air to enter into the additional length of hose 40 and causes the patient to re-breathe the exhaled air on the following breath to, thereby, effect re-breathing and to cause a change in the effective ventilation of the patient.
The change in CO.sub.2 elimination and in the partial pressure of end-tidal CO.sub.2 caused by the change in ventilation in the system of FIG. 2 can then be used to calculate the cardiac output or pulmonary capillary blood flow of the patient. Sensing and/or monitoring devices may be attached to the re-breathing ventilation circuit 30 between the additional length of hose 40 and the tubular portion 32. The sensing and/or monitoring devices may include, for example, means 44 for detecting CO.sub.2 concentration and means 46 for detecting respiratory flow parameters during inhalation and exhalation. These sensing and/or monitoring devices are typically associated with data recording and display equipment (not shown). One problem encountered in use of the conventional re-breathing system is that the volume of the deadspace provided by the additional length of hose 40 is fixed and may not be adjusted. As a result, the amount of deadspace provided in the circuit for a small adult to effect re-breathing is the same amount of deadspace available for a large adult to effect re-breathing, and the resulting changes in CO.sub.2 values for patients of different sizes or breathing capacities, derived from fixed-deadspace systems, can produce inadequate evaluation of a patient's cardiac output or pulmonary capillary blood flow. Further, the three-way valve 42 of the system is expensive and significantly increases the cost of the ventilation device.
During total re-breathing, the partial pressure of end-tidal carbon dioxide (Pet.sub.CO.sub..sub.2 ) is typically assumed to be equal to the partial pressure of carbon dioxide in the venous blood (Pv.sub.CO.sub..sub.2 ) of the patient, as well as to the partial pressure of carbon dioxide in the arterial blood (Pa.sub.CO.sub..sub.2 ) of the patient and to the partial pressure of carbon dioxide in the alveolar blood (PA.sub.CO.sub..sub.2 ) of the patient. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve.
In partial re-breathing, measurements during normal breathing and subsequent re-breathing are substituted into the carbon dioxide Fick equation. This results in a system of two equations and two unknowns (carbon dioxide content in the mixed venous blood and cardiac output), from which cardiac output or pulmonary capillary blood flow can be determined without knowing the carbon dioxide content of the mixed venous blood (CV.sub.CO.sub..sub.2 ).
Total re-breathing is a somewhat undesirable means of measuring cardiac output or pulmonary capillary blood flow because the patient is required to breathe directly into and from a closed volume of gases (e.g., a bag) in order to produce the necessary effect. Moreover, it is typically impossible or very difficult for sedated or unconscious patients to actively participate in inhaling and exhaling into a fixed volume.
Known partial re-breathing methods are also advantageous over invasive techniques of measuring cardiac output or pulmonary capillary blood flow because partial re-breathing techniques are non-invasive, use the accepted Fick principle of calculation, are easily automated, and facilitate the calculation of cardiac output or pulmonary capillary blood flow from commonly monitored clinical signals. However, known partial re-breathing methods are somewhat undesirable because they are a less accurate means of measuring the cardiac output or pulmonary capillary blood flow of non-intubated or spontaneously breathing patients, may only be conducted intermittently (usually at intervals of at least about four minutes), and result in an observed slight, but generally clinically insignificant, increase in arterial CO.sub.2 levels. Moreover, the apparatus typically employed in partial re-breathing techniques do not compensate for differences in patient size or breathing capacities. In addition, many devices employ expensive elements, such as three-way valves, which render the devices too expensive to be used as disposable units.
Thus, there is a need for adjustable deadspace re-breathing apparatus that compensate for differences in the sizes or breathing capacities of different patients, that may be employed to provide a more accurate and continuous measurement of gases exhaled or inhaled by a patient, and are less expensive than conventional re-breathing apparatus and, thereby, facilitate use of the adjustable deadspace re-breathing apparatus as a single-use, or disposable, product. There is also a need for a more accurate method of estimating the cardiac output or pulmonary capillary blood flow of a patient. ##EQU2##
where Ca.sub.CO.sub..sub.2 is the CO.sub.2 content of the arterial blood of a patient, CV.sub.CO.sub..sub.2 is the CO.sub.2 content of the venous blood of the patient, and the subscripts 1 and 2 refer to measured values before a change in ventilation and measured values during a change in ventilation, respectively. The differential form of the Fick Equation can, therefore, be rewritten as: ##EQU3##
where .DELTA.VCO.sub.2 is the change in CO.sub.2 elimination in response to the change in ventilation, .DELTA.Ca.sub.CO.sub..sub.2 is the change in the CO.sub.2 content of the arterial blood of the patient in response to the change in ventilation, .DELTA.Pet.sub.CO.sub..sub.2 is the change in the partial pressure of end-tidal CO.sub.2, and s is the slope of a CO.sub.2 dissociation curve known in the art. The foregoing differential equation assumes that there is no appreciable change in venous CO.sub.2 concentration during the re-breathing episode, as demonstrated by Capek. Also, a CO.sub.2 dissociation curve, well known in the art, is used for determining CO.sub.2 concentration based on partial pressure measurements.
In previous partial re-breathing methods, a deadspace, which may comprise an additional 50-250 ml capacity of air passage, was provided in the ventilation circuit to decrease the effective alveolar ventilation. In the present invention, a ventilation apparatus is provided with a deadspace having an adjustable volume to provide a change in ventilation for determining accurate changes in CO.sub.2 elimination and in partial pressure